1. Field of the Invention
The present invention relates to a demodulating circuit for a self-clocking-information signal. More specifically, the present invention relates to a demodulating circuit for demodulating the original digital data from a self-clocking-information (SCI) signal including a rectangle wave pulse train obtained by modulating a digital data signal of the logics one and zero such that the signal reversion periods may be of two or more predetermined different lengths.
2. Description of the Prior Art
In recording on a recording medium such as a magnetic tape the information stored in a storage means in a computer, a data processing unit, and the like, first of all information is extracted from the storage means while the same is gated as a function of a timing clock or a reading clock. The information thus extracted comprises a digital data signal of the logics one and zero and normally a timing clock and such a digital data signal are recorded in and reproduced from separate tracks of a magnetic tape. Since this approach requires two tracks (a digital data track and a timing clock track) utilization efficiency of a recording medium is poor. Therefore, for the purpose of enhancing the data capacity in a recording medium such as a magnetic tape, a self-clocking-information signal is produced through modulation of various types by combining a timing clock and a digital data signal, whereupon the self-clocking-information signal is recorded in and reproduced from a recording medium such as a magnetic tape, and the original digital data signal is demodulated from the reproduced self-clocking-information signal.
FIG. 1 shows waveforms of a self-clocking-information signal during of recording in accordance with various conventional modulation types which constitute the background of the invention. Referring to FIG. 1, the reference character T denotes a time zone corresponding to a zone of a magnetic tape in which the respective data bits are recorded. The waveform (A) shows an FM (frequency modulation) type, wherein the signal is reversed at the center of the zone for the logic one of the data bit and at the boundary between two adjacent data bits. The waveform (B) shows an MFM (modified frequency modulation) type, wherein the signal is reversed at the center of the zone for the logic one of the data bit and at the boundary between two consecutive data bits of the logic zero. The waveform (C) shows an NRZI type, wherein the signal is reversed only at the center of the zone for the logic one of the data bit. As for a signal reversion period, the FM type contains two types of signal reversion periods of 1/2T and T, the MFM type contains three types of the signal reversion periods of T, 1.5T and 2T, and the NRZI type contains an indefinite signal reversion period, inasmuch as the signal reversion period can be T, 2T, 3T, 4T, . . . depending on the data series. The present invention is directed to an improvement in a demodulating circuit of a self-clocking-information signal produced based on a modulation type such as an FM type, an MFM type, and the like, wherein the signal reversion period can be any one of a predetermined number of different periods.
Meanwhile, as a method for demodulating a conventional self-clocking-information signal, an approach has been proposed in which a self-clocking-information signal is applied to a phase locked loop to reproduced clocks, whereupon the digital data signal is demodulated from the reproduced clocks and the self-clocking-information signal. However, such a conventional demodulating circuit, suffered from a problem of a time period required for a locking time or instability of the phase locked loop in conjunction with the disorder of a reproduced self-clocking-information signal caused by a peak shift, drop out and the like of the signal on the occasion of reproduction from a magnetic tape, for example, and also suffered from problems of such as temperature characteristic variation, time dependent variation and the like of time constant components included in the phase locked loop. Therefore, such a conventional demodulating circuit employing a phase locked loop could not necessarily perform a preferred clock reproduction. Furthermore, another disadvantage was encountered that in demodulating the original digital data signal from a self-clocking-information signal using a reproduced clock a delay means is required, which adds a factor of instability.
The above described disadvantages of the conventional approaches will be described in more detail with reference to FIGS. 2 and 3. FIG. 2 is a time chart of a self-clocking-information signal as FM modulated and a reproduced clock as reproduced from the self-clocking-information signal and FIG. 3 is a time chart of a self-clocking-information signal as MFM modulated and a reproduced clock reproduced from the self-clocking-information signal. Referring to FIGS. 2 and 3, (A) each shows a waveform of the digital data, wherein the repetition period of the data bit is defined as T and a half thereto is defined as To. Referring to FIGS. 2 and 3, (B) each shows a waveform of a self-clocking-information signal obtained by FM modulating and MFM modulating the digital data (A). Referring to FIGS. 2 and 3, (C) each shows a reproduced clock reproduced by a demodulating method from the self-clocking-information signal by the use of a conventional phase locked loop. Referring to FIGS. 2 and 3, (D) each shows a waveform obtained by frequency dividing at the fall of the reproduced clock.
In demodulating a digital data signal in accordance with the conventional approaches, use is made of the fact that if and when reversion of the self-clocking-information signal (B) (of FIGS. 2 and 3) occurs in the vicinity of the rise of the reproduced clock (D) (of FIGS. 2 and 3) the data is the logic one and if and when reversion of the self-clocking-information does not occur the data is the logic zero and a gated signal obtained as the logic one for a predetermined time period after the reversion of the self-clocking-information signal is used as the data, while the signal obtained by delaying the reproduced clock for a predetermined time period is used as a reading or demodulating clock operable at the rise thereof. According to such conventional approaches, a delay means is required for delaying the signal and therefore addition of such unstable element is not preferred. Furthermore, the self-clocking-information signal reproduced from a magnetic tape and the like contains fluctuations due to instability of a magnetic tape drive mechanism, peak shift and the like. Therefore, in this case, a situation occurs in which the reproduced self-clocking-information signal (B) and the regenerated clock (C) or (D) are out of phase due to a response rate of the phase locked loop and a disadvantage is encountered during such a period that a digital data signal is eroneously demodulated.